Satisfactory analysis of the long-term or delayed myocardial actions of experimental agents under precisely controlled conditions is extremely difficult with conventional experimental preparations. Accordingly, to provide a means of investigating the long-term effects of agents on the isolated mammalian heart, we have developed a system for maintaining beating hearts from late fetal mice in organ-culture for several weeks. The system is the only currently available model for maintaining intact, stably-functioning mammalian hearts in vitro for long periods. The aim of the present program is to study the long-term myocardial effects of a variety of substrates, hormones, and pharmacological and toxicological interventions. Observations will be made on the duration of cardiac survival in vitro; histological appearance; rate and strength of beating; metabolism of fatty acids, glucose, lactate, and glycogen. Previous results indicate that this system provides an expecially useful experimental model for studying the factors that regulate net protein balance in the heart, defining the relative importance of changes in rates of protein synthesis and degradation, and elucidating possible roles for lysosomal and other hydrolytic enzymes in controlling protein breakdown. This approach provides a unique opportunity for analyzing direct, long-term actions of agents on cardiac metabolism and contraction. The studies will also test the agents' ability to enchance cardiac survival in vitro, and thus may provide information about their potential usefulness in the preservation of hearts under artificial conditions for clinical purposes.